1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method adapted for use in an image processing apparatus for converting the gradation characteristics of plural images taking a same portion of an object, and more particularly to an image processing apparatus and an image processing method adapted for use, for example in the medical radiology field or the like, in a subtraction image processing apparatus for processing plural radiological images obtained by radiological image takings of an object under different image taking conditions.
2. Related Background Art
The X-ray radiological images for medical purpose have long been obtained by converting the intensity distribution of X-ray transmitted by the human body into the intensity distribution of fluorescent light of a fluorescent member and recording such light intensity distribution directly on a silver halide-based film, but, in recent years, the radiological images are being read in the form of electrical signals by a method of forming and reading a latent energy image representing the X-ray intensity distribution in a photostimulable fluorescent member, a method of directly reading the distribution of fluorescent light generated on a fluorescent member by X-ray or a method not depending on the fluorescent light, and a digital image is obtained by digital conversion of such electrical signals.
The use of such digital image allows to improve the efficiently of filing, to implement remote diagnosis, and to achieve improvements in the diagnostic technologies and efficiency. In addition, such digital image enables various image processing, thus realizing various changes in the method of diagnosis.
Among such image processing, most common is the gradation processing. As an example, a regular conversion of the pixel values within the image allows to enhance a pattern that is not easily recognizable in the original image, into a more easily recognizable form. Also even in case the condition of radiological image taking is inappropriate or unstable, the image that would be unusable on the conventional X-ray film can be rectified to stable gradation characteristics by appropriate gradation processing.
Among such gradation processing, the averaging of histogram (frequency distribution of pixel values) is simplest and most commonly employed, but this method may be unsuitable for diagnostic purpose since it only executes straight-forward averaging by broadening concentrated portions of the frequency distribution of the pixel values in the entire image and narrowing or uniting less dense portions of the frequency distribution. The image used for diagnosis has been developed over a long history based on the conventional silver halide-based X-ray film, and, for the radiologists, the gradation characteristics (appearance of image) similar to those of the conventional X-ray film are better for the efficiency and accuracy of diagnosis. More specifically, the histogram of the image of the conventional silver halide-based X-ray film is naturally not flat but characteristically describes the area of disease by the image taking technology developed over many years, and the radiologists have been so trained as to identify the diseases based on such form of image presentation.
It is therefore desired, also in the gradation processing of the digital image, to effect conversion into such image enabling easy diagnosis in stable manner. Such objective has been attained by processing the image, obtained by a digital image taking device with characteristics significantly different from those of the conventional silver halide-based film, through a considerably complex process including empirical methods, but a first drawback lies in a fact that such process inevitably involves a generally very long processing time and cumbersome operations.
A large advantage of image digitization lies in the ease of filing and retrieval of the image. For example, it is rendered possible to promptly investigate the change in the disease in a same patient by retrieving and comparing the image filed in the past. If the image taken in the past was obtained under different image taking conditions, the difference in such image taking conditions can be canceled by applying a gradation processing. However, there is encountered a second drawback that a cumbersome process is required for substantially matching the gradation of the past image with that of the current image.
Also in case of obtaining an image corresponding to the distribution of transmittance for a radiation such as X-ray thereby showing the internal structure of an object, there are conventionally been known so-called energy subtraction process of utilizing two radiations of different energy distributions to obtain two images corresponding to the distribution of transmittance, based on a fact that a specified constituent in the interior of the object has specific radiation transmittance (or absorbing characteristics for the energy of radiation), then digitizing these images and effecting a calculation between these images to extract or erase the specified constituent in the interior of the object. In the processing of medical X-ray images, such process is considered extremely useful for diagnostic purpose, such as erasure or extraction of a bone portion.
FIG. 1 shows a conventional energy subtraction apparatus, in which provided are a tube 101 for generating X-ray; a human body 102 constituting the object; a CCD 103 constituting a solid-state image pickup device for separating the distribution of radiation into pixels thereby obtaining an electrical signal; an A/D converter 104 for converting an analog electrical signal into digital values; an offset/gain correction unit 105 for correcting the fluctuation in the offset-gain of the CCD 103; a logarithmic conversion unit 106 for converting digital values, proportional to the obtained radiation intensity, into logarithmic values; memories 107, 108 serving to store the image data and connected to a signal bus 111; a central processing unit (CPU) 109 capable of access to the memories 107, 108 through the bus 111; and a program memory medium 110 such as a FD, a HD or a MOD in which the process sequence is stored as a program. The final image obtained after energy subtraction is stored in a memory 112 connected to the bus 111.
In the following there will be explained the function of the apparatus described above. At first the X-ray tube 101 emits X-ray with a first energy distribution, and the obtained image is converted in the CCD 103 into an electrical signal, which is converted by the A/D converter 104 into digital values. After the correction in the offset/gain correction unit 105 and the conversion into values corresponding to logarithmic values in the logarithmic conversion unit 106, the digital values are once stored in the memory 107. Then the X-ray tube 101 emits the X-ray with a second energy distribution, and the image obtained by logarithmic conversion in a similar process is stored in the memory 108.
Then considered is a pixel in the logarithmically converted two images obtained in the above-described manner, and such pixel is assumed to receive X-ray that has transmitted a bone and a soft tissue in the human body.
In the first energy distribution, the intensity H1 of transmitted radiation is given by:
H1=I1exp(xe2x88x92xcexc1t1)exp(xe2x88x92xcexc2t2)xe2x80x83xe2x80x83(1)
wherein xcexc1, xcexc2 are radiation transmittances of respectively bone and soft tissue, t1, t2 are thicknesses thereof, and I1 is the intensity of first incident radiation in this portion.
Similarly the intensity H2 of transmitted radiation in the second energy distribution is given by:
H2=I2exp(xe2x88x92xcexcxe2x80x21t1)exp(xe2x88x92xcexcxe2x80x22t2)xe2x80x83xe2x80x83(2)
wherein xcexcxe2x80x21, xcexcxe2x80x22 are radiation transmittances of respectively bone and soft tissue, t1, t2 are thicknesses thereof, and I2 is the intensity of second incident radiation in this portion.
As digital data proportional to these logarithmic conversions are stored as pixel values, the corresponding pixel data h1, h2 are given by:
h1=K(xcexc1t1+xcexc2t2)+C1xe2x80x83xe2x80x83(3)
h2=K(xcexcxe2x80x21t1+xcexcxe2x80x22t2)+C2xe2x80x83xe2x80x83(4)
wherein K, C1 and C2 are constants.
These equations (3) and (4) are considered as simultaneous equations, and, if the transmittances or the ratio thereof is known, the thickness t1 of the bone and that t2 of the soft tissue can be obtained as sums with a constant offset value. Since such offset value is substantially constant over the entire image, a suitable offset value is added or the image is inverted in order to render the image more visible or to eliminate negative pixel data, and the energy subtraction image thus obtained is stored in the memory 112.
It is naturally possible also to obtained images under plural conditions instead of two conditions, and to suitably process such plural images to obtain a single image in which one or plural portions are erased or enhanced.
The diagnosis with the X-ray image has a long history, and, based on the experience in such history, the radiologists are accustomed to the ordinary image density distribution specific to the human body and are trained for diagnosis utilizing such distribution. Consequently the image obtained by energy subtraction, for example showing soft tissues such as blood vessels or tumor only by erasure of the bones, is merely the solution of the simultaneous equations (3) and (4) and is different from the original X-ray transmittance. Therefore the gradation characteristics of the image are different from those to which the radiologists are used, and the efficiency of diagnosis may be deteriorated.
Furthermore, the image obtained by energy subtraction shows different gradation characteristics based on the image taking conditions (two X-ray energy distributions), so that stable gradation independent of the image taking condition cannot be obtained by mere correction of the offset.
An object of the present invention is to provide an image processing apparatus and an image processing method capable of solving the above-described drawbacks.
Another object of the present invention is to provide an image processing apparatus and an image processing method capable of converting the gradation characteristics of plural images taking a same part of the object into an image with desired gradation characteristics in a simple manner.
The above-mentioned objects can be attained, according to an embodiment of the present invention, by an image processing apparatus capable of varying the gradation characteristics of image, comprising histogram distribution characteristic detecting means for inputting an image signal and detecting the histogram distribution characteristic of the pixel values constituting the image represented by the inputted image signal, and histogram distribution characteristic converting means for converting the histogram distribution characteristic, detected by the histogram distribution characteristic detecting means, into an arbitrary histogram distribution characteristic.
Still another object of the present invention is to provide an image processing apparatus and an image processing method, capable of converting the gradation characteristics of an image obtained by a subtraction image processing apparatus so as to be similar to the gradation characteristics of the originally taken image.
The above-mentioned object can be attained, according to an embodiment of the present invention, by an image processing apparatus for processing plural images obtained under mutually different image taking conditions, comprising differential image signal generating means for inputting plural image signals corresponding to plural images obtained under mutually different image taking conditions and generating a differential image signal corresponding to a differential image of the images represented by the inputted plural image signals, and histogram distribution characteristic converting means for inputting the differential image signal generated by the differential image signal generating means and converting the histogram distribution characteristic of the pixels values, constituting the differential image represented by the inputted differential image signal, into an arbitrary histogram distribution characteristic.
Still other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description of the embodiments, which is to be taken in conjunction with the attached drawings.